This document discusses accommodation and presbyopia. It defines accommodation as the ability of the eye to change focus from distant to near objects by changing the shape of the lens. Accommodation decreases with age in a process called presbyopia as the lens becomes less flexible. Presbyopia symptoms include difficulty seeing close objects clearly and can be treated with reading glasses, bifocal glasses, or multifocal contact lenses to restore near vision. The document also covers topics like ciliary muscle function, amplitude of accommodation measurement, and factors affecting presbyopia.

1. OPT 203-GEOMETRIC OPTICS -2
UNIT–IV

2. ACCOMMODATION
• Accommodation, the mechanism by which the eye changes focus from distant to
near images, is produced by a change in lens shape resulting from the action of the
ciliary muscle on the zonular fibers
• The lens substance is most malleable during childhood and the young adult years,
progressively losing its ability to change shape with age.
• After approximately 40 years, the rigidity of the lens nucleus clinically reduces
accommodation because the sclerotic nucleus cannot bulge anteriorly and change
its anterior curvature as it could before

3. According to the classic theory of von Helmholtz, most of the accommodative
change in lens shape occurs at the central anterior lens surface.
The central anterior capsule is thinner than the peripheral capsule, and the
anterior zonular fibers insert slightly closer to the visual axis than do the
posterior zonular fibers, resulting in a central anterior bulge with
accommodation.
The posterior lens surface curvature changes minimally with accommodation.
The central posterior capsule, which is the thinnest area of the capsule, tends to
bulge posteriorly to the same extent regardless of zonular tension

4. The ciliary muscle is a ring that, upon contraction, has the opposite
effect from that intuitively expected of a sphincter. When a sphincter
muscle contracts, it usually tightens its grip.
when the ciliary muscle contracts, the diameter of the muscle ring is
reduced, thereby relaxing the tension on the zonular fibers and
allowing the lens to become more spherical.
Thus, when the ciliary muscle contracts, the axial thickness of the lens
increases, its diameter decreases, and its dioptric power increases,
producing accommodation.
When the ciliary muscle relaxes, the zonular tension increases, the lens
flattens, and the dioptric power of the lens decreases.
The accommodative response may be stimulated by the known or
apparent size and distance of an object or by blur, chromatic
aberration, or a continual oscillation of ciliary tone.

5. Accommodation is mediated by the parasympathetic fibers of
cranial nerve III (oculomotor).
Parasympathomimetic drugs (eg, pilocarpine) induce
accommodation, whereas parasympatholytic medications (eg,
atropine) block accommodation.
Drugs that relax the ciliary muscle are called cycloplegics

6. The amplitude of accommodation is the amount of change in the
eye’s refractive power that is produced by accommodation. It
diminishes with age and may be affected by some medications
and diseases. Adolescents generally have 12–16 D of
accommodation, whereas adults at age 40 have 4–8 D. After age
50, accommodation decreases to less than 2 D. It is thought that
hardening of the lens with age is the principal cause of this loss
of accommodation, which is called presbyopia. Research is under
way into other possible contributing factors in presbyopia, such
as changes in lens dimensions, in the elasticity of the lens
capsule, and in the geometry of zonular attachments with age.

9. FAR POINT, NEAR POINT, RANGE AND AMPLITUDE OF ACCOMMODATION
• The nearest point at which small objects can be seen clearly is called near
point or punctum proximum and the distant (farthest) point is called far
point or punctum remotum. The distance between the near point and the
far point is called range of accommodation. The difference between the
dioptric power, needed to focus at near point(P) and to focus at far point
(R) is called amplitude of accommodation(A). Thus A= P-R

10. Stimulus for accommodation
• There is no single important stimulus for accommodation. All of the following
factors must be responsible in elicting the appropriate accommodative response.
1. Image blur
2. Apparent size and distance of object
3. Chromatic aberrations
4. Oscillation of accommodation
5. Scanning movements of the eye

11. REACTION TIME
• Reaction time refers to the time lapse between the presentation of an
accommodative stimulus and occurrence of the accommodative response.
• 1. Average reaction time for ‘far to near’ accommodation is 0.64
seconds.
• 2. Average reaction time for ‘near to far’ accommodation is 0.56
seconds.
• 3. Reaction time for accommodation is considerably larger than that for
the contraction of the pupil to light (0.26-0.30seconds). Reaction time of
convergence response is about 0.20 seconds

13. ASSESSMENT OF ACCOMMODATION
• accommodation is a unique mechanism by which our eyes can even focus
the diverging rays coming from a near object on the retina in a bid to see
clearly. Assessment of accommodation is of great diagnostic value in cases
of incomitant strabismus of non-paralytic origin. Assessment of amplitude
of accommodation can be made either by measurement of NPA or by use
of minus lenses.

14. MEASUREMENT OF NPA
NPA is measured using a near point rule such as RAF rule or prince’s rule. To
determine the NPA, a sliding target with 6/9 letters, numbers or fine lines is
moved from or towards the eye until the closest point is found at which it still
can be seen clearly.
During the examination, the patient has to wear his or her full optical
refractive correction. The NPA is determined first for each eye separately and
then for both eyes together. The NPA is measured in centimeters marked on
one side of the instrument bar. The side of bar marked in dioptres will
indicate the amplitude of accommodation in dioptres. The third side of the
bar shows the age corresponding to the accommodation.
e.g. if the patient reports that the point appears blurred at 25 cm, the dioptric
markings will show +4.0 D and the age of 40 years.

15. RAF Rule (Royal Air Force Rule)

16.  Measurement of amplitude of accommodation using minus
lenses
• This test is also performed first for each eye separately and then for both
eyes together, and during examination the patient has to wear his or her full
refractive correction. The patient is asked to fixate 6/60 symbol at a
distance of 6m, and minus lenses of progressively increasing power are
added before the eye till one can see the target clearly. The power of this
minus lens is equivalent to the amplitude of accommodation in dioptres.

17.  PRESBYOPIA
• Presbyopia is the normal loss of near focusing ability that occurs with age.
Most people begin to notice the effects of presbyopia sometime after age 40,
when they start having trouble seeing small print clearly — including text
messages on their phone.
• You can't escape presbyopia, even if you've never had a vision problem before.
Even people who are nearsighted will notice that their near vision blurs when
they wear their usual eyeglasses or contact lenses to correct distance vision.

19.  Presbyopia symptoms
• When you become presbyopic, you either have to hold your smartphone
and other objects and reading material (books, magazines, menus, labels,
etc.) farther from your eyes to see them more clearly.
• when you move things farther from your eyes they get smaller in size, so
this is only a temporary and partially successful solution to presbyopia.
• If you can still see close objects pretty well, presbyopia can cause
headaches, eye strain and visual fatigue that makes reading and other
near vision tasks less comfortable and more tiring

20.  What causes presbyopia?
• Presbyopia is an age-related process. It is a gradual thickening and loss of
flexibility of the natural lens inside your eye.
• These age-related changes occur within the proteins in the lens, making the
lens harder and less elastic over time. Age-related changes also take place
in the muscle fibers surrounding the lens. With less elasticity, it gets
difficult for the eyes to focus on close objects.

21. • PRESBYOPIA TREATMENT
 Eyeglasses
• Eyeglasses with progressive lenses are the most popular solution for presbyopia for most people over age 40. These line-free multifocal
lenses restore clear near vision and provide excellent vision at all distances.
• Another presbyopia treatment option is eyeglasses with bifocal lenses, but bifocals provide a more limited range of vision for many
people with presbyopia.
• It's also common for people with presbyopia to notice they are becoming more sensitive to light and glare due to aging changes in their
eyes. Photochromic lenses, which darken automatically in sunlight, are a good choice for this reason.
• Reading glasses are another choice. Unlike bifocals and progressive lenses, which most people wear all day, reading glasses are worn
only when needed to see close objects and small print more clearly.
• If you wear contact lenses, your eye doctor can prescribe reading glasses that you wear while your contact lenses are in. You may
purchase reading glasses at an eyewear retail store, or you can get higher-quality versions prescribed by your eye doctor.

22.  Contact Lenses
• People with presbyopia also can opt for multifocal contact lenses,
available in gas permeable or soft lens materials.
• Another type of contact lens correction for presbyopia
is monovision, in which one eye wears a distance prescription, and
the other wears a prescription for near vision. The brain learns to
favor one eye or the other for different tasks.

23. • Magnification Formulae
• Linear Magnification
• The linear magnification produced by a spherical lens can be calculated from the basic formula:
• Linear magnification= I/O=V/U
• where I is the image size, O is the object size, v is the distance of the image from the principal plane,
• and u is the distance of the object from the principal plane (Fig. 5.9).
•

25. • Magnification
• Traditionally, three types of magnification are discussed: relative distance
magnification, relative size magnification, and angular magnification.
• a. Relative Distance Magnification
• The easiest way to magnify an object is to bring the object closer to the eye.
By moving the object of regard closer to the eye, the size of the image on the
retina is enlarged.
• Children with visual impairments do this naturally. Adults will require reading
glasses to have the object in focus.
• Relative Distance Magnification = r/d where r = reference or original working
distance and d = new working distance
• Example
• Original working distance = 40cm
• New working distance = 10cm
• Relative Distance Magnification (RDM) = 40/10 = 4x

26. • Magnification occurs because the lens strength requires the individual
using them to hold things closer to have the object in focus.
• b. Relative Size Magnification
• Relative size magnification enlarges the object while maintaining the same
working distance, for instance, as observed with large print.
• Relative Size Magnification = S2/S1 where S1 = original size and S2 = the
new size
• Example
• Original size = 1M
• New size = 2M
• Relative Size Magnification (RSM) = 2/1 = 2x

27. • Depth of Focus
• Depth of focus describes the image location range where the image is
clear when focused by an optical system. Outside this range, the image
will be significantly blurry. However, within this few millimeters range, the
image appears quite sharp.
• Depth of Field
• Depth of field is the same principle for objects as the depth of focus is for
images. When an optical system such as the camera is focused on an
object, nearby objects are also, in focus, inside the camera’s depth of field.
Objects outside of the depth of field will be out of focus.

31. THANK YOU